There have been quite a few posts related to issues installing the VirtualBox Extension Pack for both Windows and Linux hosts.http://forums.virtualbox.org/viewtopic.php?p=11262&sid;=334fb962995ae00d32bb8988192f701chttp://www.virtualbox.org/ticket/7899http://www.virtualbox.org/ticket/7972http://blogs.oracle.com/wim/2010/12/oracle_vm_virtualbox_40_extens.htmlThe error message given is very cryptic: “Failed to install the Extension Pack” NS_ERROR_FAILURE (0x80004005)Weird.In digging through the above posts I found tidbits of the solution that I was able to put together. I’m currently running a Ubuntu 10.10 x64, so here’s how I solved the problem and installed the extension pack. Give the VBoxExtPackHelperApp execute permissions and then run the install from the command line.sudo chmod 744 /usr/lib/virtualbox/VBoxExtPackHelperAppsudo /usr/lib/virtualbox/VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.0.4-70112.vbox-extpack